For reasons of comfort and performance, powershift transmissions are increasingly used in utility vehicles. For this, dual clutch transmissions that can be shifted with no traction force interruption are particularly attractive. Transmission structures of dual-clutch transmissions for utility vehicles are known for example from DE 10 2005 033 027 A1 and DE 10 2006 054 281 A1. A method for controlling shift processes in a dual-clutch transmission is described for example in DE 10 2004 039 273 A1.
Such automated shift transmissions comprise an intermediate transmission or a number of intermediate transmission groups connected one after another, and if necessary also a downstream planetary transmission. The gears are divided into two groups, each group being associated with one clutch of the dual clutch system, by virtue of which a sequential shift sequence almost free from traction force interruptions can be carried out. In a conventional manner the force flow of the gears can pass via a drive input shaft and a drive output shaft, or it can meander through the transmission by way of repeatedly changing shafts. Compared with pure powershift automatic transmissions of planetary design, automated shift transmissions have efficiency and cost advantages. However, as the number of gears increases, so also do the overall size, the structural complexity and thus also the production costs.
Since as a rule, depending on their field of use, to obtain a particular transmission spread utility vehicle transmissions need a relatively large number of gears for efficient operation, it is expedient also to consider mixed forms, so-termed partial dual-clutch transmissions, in which, besides a powershift transmission or partial transmission with a dual clutch, there is provided a conventional transmission section, i.e. one that is shifted with traction force interruption, or a downstream transmission that shifts with traction force interruption.
In addition, partial dual-clutch transmissions can advantageously be combined by way of a transfer box or axle transmission with long gear ratios on the rear axle of utility vehicles, which enable fuel-saving and low-emission operation of the drive engine at relatively low rotation speeds. However, owing to the rather low torque reserve in such an operating range this tends to lead to a further increase of the in any case high shifting frequency in utility vehicle transmissions and hence also to a larger number of shift processes with traction force interruption in the conventional part of the transmission.
Upshift processes with traction force interruption are fundamentally always problematic since, to reduce the torque losses, the aim in modern internal combustion engines is to have speed reversion or speed reduction gradients which are as flat at possible. In upshift processes this leads either to more intensive slipping operation at the two clutches of the dual clutch, which translates into higher clutch wear particularly when shift processes are frequent as they are in utility vehicles, or to longer shifting times and hence to longer, unfavorable traction force interruptions since the synchronizing of the clutch in each case takes a longer time.
To accelerate the speed reversion of the internal combustion engine, although an existing engine brake of the utility vehicle can be activated during the shifts, this interferes with driving comfort because of clearly perceptible engagement and disengagement times and operating noise and also results in premature wear because of the large number of shift cycles. Thus, speeding up the speed adaptation during shift processes in the conventional transmission section of partial dual-clutch transmissions by means of the engine brake is rather problematic.
Synchronization, i.e. the speed adaptation of the components to be shifted during a shift process, can also be assisted by an existing transmission brake. In the case of unsynchronized claw-type shifting elements the synchronizing in upshifts can be done completely by means of the transmission brake. However, such an additional brake needs to be designed in accordance with its required power and shifting frequency, and can therefore occupy a relatively large amount of structural space and be correspondingly costly. Thus, speeding up the speed adaptation during shift processes in the conventional transmission section of the partial dual-clutch transmissions exclusively by means of a transmission brake is not optimal.